1. Field of the Invention
The invention relates generally to motors attached to a drill string and used for drilling an earth formation. More specifically, the invention relates to a turbine motor powered by the flow of drilling fluid.
2. Background Art
Drilling motors are commonly used to provide rotational force to a drill bit when drilling earth formations. Drilling motors used for this purpose are typically driven by drilling fluids pumped from surface equipment through the drill string. This type of motor is commonly referred to as a mud motor. In use, the drilling fluid is forced through the mud motor(s), which extract energy from the flow to provide rotational force to a drill bit located below the mud motors. There are two primary types of mud motors: positive displacement motors (“PDM”) and turbodrills.
A PDM is based on the Moineau principle. Drilling fluid is forced through a stator. An eccentric rotor is located inside the stator. Drilling fluid circulating through the stator imparts a rotational force on the rotor causing it to rotate. This rotational force is transmitted to a drill bit located below the PDM.
A turbodrill uses one or more stages to provide rotational force to a drill bit. A typical prior art turbodrill is shown in FIG. 1. In FIG. 1, a turbodrill 8 is connected to a drill string 4. A drill bit 3 is connected to a shaft 1 on a lower end of the turbodrill 8. In operation, drilling fluid (not shown) is pumped through the drill string 4 until it enters the turbodrill 8. The flow path of the drilling fluid through the turbodrill 8 is indicated by arrows. When the drilling fluid enters the turbodrill, the flow is substantially in the axial direction in line with the axis -A- of the turbodrill 8. The drilling fluid is diverted from the center of the turbodrill 8 to an outer radial position of the turbodrill 8. The drilling fluid then passes through a stator 6, which is rotationally fixed relative to the housing 2 and the drill string 4. A plurality of curved stator vanes 9 are positioned around stator 6. As the drilling fluid passes through the stator 6, it accelerates and the flow direction is changed by a selected angle, which is typically referred to as the swirl angle. The resulting flow direction is helical with respect to the axis -A-.
After passing through the stator 6, the drilling fluid passes through the rotor 7. A plurality of curved rotor vanes 10 are positioned around the rotor 7. The rotor vanes 10 are curved to direct flow in an opposing direction to the helical flow resulting from the stator 6. The rotor vanes 10 are shaped similarly to an airfoil so that the drilling fluid passes efficiently through the rotor 7. The energy required to change the rotational direction of the drilling fluid is transformed into rotational and axial (thrust) force. This energy transfer is seen as a pressure drop in the drilling fluid. The thrust is typically absorbed by thrust bearings (not shown). The rotational force causes the rotor 7 to rotate relative to the housing 2. The rotor 7 rotates the shaft 1, which may be connected to a drill bit 3.
FIG. 1 also illustrates the use of multiple “stages” in a turbodrill 8. A stage includes a stator blade 9 and a rotor blade 10, each having an arrangement of blades thereon. The rotor blades 10 of each stage are typically attached to the same rotor 7. Each stage generates an amount of power and torque, and results in a corresponding pressure drop for a given flow rate. Multiple stages are stacked coaxially until the desired power and torque is achieved. Because a pressure drop results from each stage, the total pressure drop must be considered based on the pumping ability of the pumps (not shown) used to convey the fluid downhole. The stacking of stages also increases the overall length of the tool.
What is still needed are improved turbodrills. Desired improvements may include shorter length, increased efficiency, lower axial thrust, power curves with wider operating regions, and the ability to be used with mud of various density and viscosity.